1. Field of the Invention
The invention relates generally to acoustic transducers. More particularly, this invention relates to improved acoustic sources and sensors for subsurface applications.
2. Background Art
In the oil and gas industry, subsurface formations are typically probed by well logging instruments to determine the formation characteristics. Among these instruments, sonic tools have been found to provide valuable information regarding subsurface acoustic properties, which may be used to produce images or derive related characteristics for the formations.
Acoustic waves are periodic vibrational disturbances resulting from acoustic energy that propagates through a medium, such as a subsurface formation. Acoustic waves are typically characterized in terms of their frequency, amplitude, and speed of propagation. Acoustic properties of interest for formations may include compressional wave speed, shear wave speed, borehole modes, and formation slowness. Additionally, acoustic images may be used to depict borehole wall conditions and other geological features away from the borehole. These acoustic measurements have applications in seismic correlation, petrophysics, rock mechanics and other areas.
Recordings of acoustic properties as functions of depth are known as acoustic logs. Information obtained from acoustic logs may be useful in a variety of applications, including well to well correlation, porosity determination, determination of mechanical or elastic rock parameters to give an indication of lithology, detection of over-pressured formation zones, and the conversion of seismic time traces to depth traces based on the measured speed of sound in the formation.
Sonic logging of earth formations entails lowering an acoustic logging instrument or tool into a borehole traversing the formation. The instrument typically includes one or more acoustic sources (i.e., a transmitter) for emitting acoustic energy into the subsurface formations and one or more acoustic sensors or receivers for receiving acoustic energy. The transmitter is periodically actuated to emit pulses of acoustic energy into the borehole, which travel through the borehole and into the formation. After propagating through the borehole and formation, some of the acoustic energy travels to the receivers, where it is detected. Various attributes of the detected acoustic energy are subsequently related to subsurface or tool properties of interest.
FIG. 1 shows a conventional downhole sonic tool. The tool 10 is shown disposed in a borehole 12 traversing an earth formation 20. The borehole 12 is typically filled with a drilling fluid 14 (“mud”) that is used during the drilling of the borehole. The tool 10 is generally implemented in a tubular 13 support, which in the case of a drill collar includes an internal passage 13A for drilling fluid 14 to reach a mud motor and/or a drill bit at the bottom of a drill string (not shown) as known in the art. The logging tool 10 includes one or more acoustic transmitters 16 and a plurality of acoustic receivers 18 disposed on the tubular 13. The receivers 18 are shown spaced apart from each other, along the longitudinal axis of the tool 10, at a selected distance h. One of the receivers 18 closest to the transmitter 16 is axially spaced there from by a selected distance a. The tool 10 also houses one or more conventional computer modules 21 including microprocessors, memory, and software to process waveform signal data as known in the art. As also known in the art, the computer module(s) 21 can be disposed within the instrument, at the earth surface, or combined between the two as shown in FIG. 1. Acoustic energy waves 22 are shown propagating in the borehole. Conventional sonic downhole tools are described in U.S. Pat. Nos. 5,852,587, 4,543,648, 5,510,582, 4,594,691, 5,594,706, 6,082,484 6,631,327, 6,474,439, 6,494,288, 5,796,677, 5,309,404, 5,521,882, 5,753,812, RE34,975 and 6,466,513.
Conventional acoustic tools are equipped with acoustic transducer elements, such as piezoelectric elements. In general, an acoustic transducer converts energy between electric and acoustic forms and can be adapted to act as a source or a sensor. Acoustic transducers are typically mounted on the body of the logging tool as shown in FIG. 1. Conventional sonic sources and sensors used in downhole tools are described in U.S. Pat. Nos. 6,466,513, 5,852,587, 5,886,303, 5,796,677, 5,469,736 and 6,084,826. For various reasons, including space constraints, these transducers typically have multiple components compacted into a package mounted on the tool with the front-end electronics and circuitry disposed remotely from the transducer elements.
Acoustic transducer devices have also been incorporated in configurations using printed circuit boards (PCBs). U.S. Pat. No. 6,501,211 describes an ultra-sonic transducer implemented in a PCB for attachment to bolt heads. The proposed transducers are coupled to a remote computer for identification of the bolts using the transducer. U.S. Pat. No. 4,525,644 describes mechanisms using piezoelectric devices located next to PCB connection pads to increase engagement forces between the connection pads and connectors. EP 1467060 A1 describes flexible piezoelectric transducers for use with downhole tools to telemeter acoustic signals through the tools. Drawbacks of these conventional acoustic transducer systems include poor sensitivity and a need for bulky electronics packages (e.g., large preamplifier stages) disposed elsewhere.
It is desirable to have improved acoustic transducers with integrated electronics and processing means without sacrificing performance and sensitivity.
SUMMARY OF INVENTION
One aspect of the invention provides an acoustic transducer assembly for subsurface use. The assembly comprising a frame; an acoustic transducer element disposed on the frame; and an electronics module disposed on the frame and linked to the acoustic transducer element; wherein the electronics module is adapted to process a signal associated with the transducer element.
One aspect of the invention provides an acoustic transducer assembly for subsurface use. The assembly comprising a frame; an acoustic transducer element disposed on the frame; an electronics module disposed on the frame and linked to the acoustic transducer element; and the electronics module adapted to digitize a signal associated with the transducer element; wherein the transducer element and the electronics module are covered with a sealing material to protect against external fluids.
One aspect of the invention provides an acoustic transducer assembly for subsurface use. The assembly comprising a disc-shaped acoustic transducer element having a first surface opposite a second surface; an electronics module coupled to the second surface of the transducer element and adapted to process a signal associated with said acoustic transducer element; the electronics module having at least one signal lead coupled thereto; acoustic damping material disposed around the electronics module and the acoustic transducer element; wherein the acoustic transducer element, the electronics module, and the damping material are enclosed within a sealing material leaving the at least one lead exposed.
One aspect of the invention provides an acoustic transducer assembly for subsurface use. The assembly comprising an elongated planar frame; an acoustic transducer element disposed on the frame; and an electronics module disposed on the frame and linked to the acoustic transducer element; wherein the electronics module is adapted to digitize a signal associated with the transducer element.
One aspect of the invention provides a method for assembling an acoustic transducer. The method including disposing an acoustic transducer element on frame means; disposing an electronics module on the frame means; linking the acoustic transducer element to the electronics module, the electronics module adapted to digitize a signal associated with the transducer element; and covering the transducer element and the electronics module with a sealing material not including liquids